The present invention relates to a belt-operated false-twisting unit.
There has been known a belt-operated false-twisting unit having two belts extending across each other and urged against each other at the intersection, the belts being drivable to run in opposite directions for nipping a yarn at the intersection to false-twist the yarn.
More specifically, as shown in FIG. 8, the two belts B1, B2 extend across each other at equal angles .theta. to a path of travel of a yarn Y to nip the yarn at the intersection to thereby false-twist the yarn. When the belts B1, B2 travel at the same speed, and the belts and the yarn run at a relative speed V, the speed V1 of travel of the belts in the direction of travel of the yarn is expressed as V1=V cos .theta., and the yarn is fed along under a force proportional to the speed V1. The speed V2 of travel of the belts in a direction normal to the direction of feed of the yarn is given as V2=V sin .theta., and the number of turns of twist is proportional to the speed V2. When the yarn is to be twisted, the force with which the yarn is urged to become untwisted should be overcome by another force which is determined by the nipping pressure given by the belts and the coefficient of friction between the belts and the yarn.
The force acting perpendicularly on the yarn, that is, the twisting force, the number of twists, the forces acting in the direction of travel of the yarn, that is, the feeding force and tensioning force, are naturally subjected to changes if the foregoing factors vary. However, the speed V of travel of the belts and the angle .theta. of intersection of the belts are normally set as fixed values during operation, and the selected nipping pressure by the belts and the coefficient of friction between the belts and the yarn will not vary to a great extent within a short period of time though they change somewhat with time. It follows therefore that the number of twists per unit length of the yarn, or the tensioning forces on the twisting and untwisting sides should remain substantially constant.
The actual yarn however is subjected to changes in the number of twists or the tensioning forces. Such variations in the number of twists or the tensioning forces are believed to be caused by displacement or vibration of the yarn in the lateral direction while the yarn is travelling, changes in the yarn nipping position due to movement of belts axially of pulley shafts, that is, changes in the length of the yarn being nipped. More specifically, the length l of the yarn Y being nipped in FIG. 8 is determined by the width L of the belts B1, B2 and the angle .theta., and can be defined as l=L/sin .theta.. When the yarn Y is displaced laterally to the left or right, or either the belt B1 or B2 is displaced in the axial direction of a pulley P1 or P2, the length of the yarn being nipped becomes smaller than the maximum length l of the yarn being nipped. At this time, the pressure under which the yarn is nipped between the belts tends to be reduced, and the belts and the yarn are liable to slip on each other, with the result that the twisting force and the yarn feeding force will be reduced.
To take the foregoing into account, it has been customary practice to place the yarn as half-twisted in a position displaced from the center of the intersection of the belts, and allow the yarn to the fully twisted position or the position in which the length of the yarn being nipped is at maximum as illustrated in FIg. 8. This indicates that the smaller the length of the yarn being nipped, the smaller the number of twists. Furthermore, as the force with which the yarn is fed in the direction of travel is reduced, the tension of the yarn on the untwisting side is increased.
The variations in the length of the yarn being nipped in the false-twisting unit during operation thereof largely affect the number of twists and the yarn tension, particularly on the untwisting side, with the consequences that ununiform false-twisted yarns will be produced and the quality of yarns produced is reduced.